The present invention relates to Vertical External Cavity Surface Emitting Laser devices (VeCSEL), and more specifically to the generation of a coherent-single-frequency vortex mode or a bi-frequency vortex mode with such devices.
The field of the invention is, but not limited to, photonic applications, and more precisely optical handling of microscopic particles, atoms manipulation, sub-diffraction microscopy and quantum information processing and communication.
Helically phased beams, also called vortex beams, exhibit a helical wave-front that varies in a corkscrew-like manner along the beam's direction of propagation, with a Poynting vector that follows a spiral trajectory around the beam axis. This behavior is described by an azimuthal phase term eiMθ in the field expression, where the factor M is known as the topological charge which can be both positive and negative integer value depending on the direction of rotation of the wave-front. This factor also signifies that the phase structure contains M intertwined helices. The transverse intensity profile in such beams looks like a ring of light with dark core at the centre, known as doughnut-shaped modes, and most commonly assumed to belong to the Laguerre-Gaussian or Bessel-Gaussian eigen-modes basis.
Several well-known methods have been developed to generate beams with helical wave-front, summarized into two distinct groups:                The first one consists in manipulating light by using external mode-transforming optics, for example using a pair of cylindrical lenses to transform Hermite-Gaussian modes into desired Laguerre-Gaussian (LG) modes with helical phase structure, or using spatial light modulators or spiral phase plates in order to convert a conventional laser beam with parallel wave-front into an exotic one carrying an Orbital Angular Momentum (OAM), as disclosed in U.S. Pat. No. 6,995,351 B2. However, the inconvenient is inherent to pixilation and non-linearity imperfection in such optics that leads to deterioration of the spatial coherence of the beam and the conversion efficiency. Moreover the power is limited by the damage threshold of the liquid crystals in such devices and beam transformation methods are generally cumbersome and need rigorous optical alignments, which makes the beam quality dependent on the experiment conditions.        The second approach consists in direct generation of the vortex laser beam, i.e. generating the helical wave-front inside the optical cavity by selecting one mode of the LG0Mset of modes thanks to modal net gain competition (thanks to spatial hole burning (SHB), a non-linear mode competition process). It is done for example by two orthogonal modes intracavity locking, or by using a ring-shaped pump beam. However, the inconvenient is that both methods need intracavity elements (absorbers, apertures, Brewster window, etalon . . . etc) which leads to a sophisticated alignment procedure and affect the spatial coherence of the beam. Furthermore, the selection of the wave-front direction of rotation is either not definite or understood.        
The aim of the present invention is to fulfil the previously mentioned problems and further to lead to some other advantages.
It is an object of the invention to provide direct laser generation of high coherence rotary-symmetrical transverse modes.
It is another object of the present invention to provide laser sources carrying an Orbital Angular Momentum with controlled charge and sign.
It is another object of the invention to provide stable, robust and controlled lasing effect of a single vortex mode.
It is another object of the present invention to provide laser sources achieving high-power operations.
It is another object of the invention to provide laser sources with an integrated device.
It is another object of the invention to reduce costs for fabricating such devices.